Recently, a high-pressure discharge lamp such as a metal halide lamp is becoming widespread as a light source of a lighting apparatus that is used as a downlight, a spotlight, or the like in commercial facilities for example. Also, store spaces especially needs only “light”, and do not hope lighting apparatuses to be brought into prominence. That is, there arises a strong demand for size reduction of lighting apparatuses.
Also, there arises a demand for ease of maintenance such as easy lamp replacement. According to a conventional lighting apparatus that includes a compact metal halide lamp having a double-tube structure of an arc tube and an outer tube, a front glass is attached to a mirror part constituting the lighting apparatus so as to cover an opening of the mirror part via which light is emitted (light extraction part). This front glass is attached because in order to, even if the arc tube and the outer tube covering the arc tube are both broken, prevent scattering of broken pieces of the arc tube and the outer tube. However, the lighting apparatus with such a structure has a problem that lamp replacement is troublesome in attachment and detachment of the front glass. On the other hand, there has been known a lighting apparatus including an explosion preventive sleeve provided between an arc tube and an outer tube. According to the lighting apparatus with such a structure, a front glass is unnecessary. However, a lamp itself instead is increased in size compared with a metal halide lamp having a double-tube structure, and accordingly cannot satisfy the strong demand for size reduction described above.
Also, there has been proposed a compact metal halide lamp having a triple-tube structure of an arc tube, an inner tube, and an outer tube (for example, Patent Literatures 1 and 2). In the case where such a metal halide lamp is used for a lighting apparatus, a front glass is unnecessary because of the lamp having the triple-tube structure. This leads to an excellent maintenance property.
Furthermore, this provides an advantage that since the lamp itself is compact, the lighting apparatus can be also downsized.
Commercialization of a metal halide lamp having such a triple-tube structure provides a metal halide lamp having an outer tube whose outer diameter is approximately 20 [mm] and a rated lamp wattage of 35 [W], 70 [W], or the like.